A color printing system for printing a color image in association with a computer is known as a technique that utilizes a color conversion table for transforming continuous tone color-specification data between different color-specification spaces.
The color image in a computer expresses tone by the three primary colors of red (R), green (G) and blue (B) for every pixel of pixels arrayed in horizontal and vertical directions. A color printer, on the other hand, prints in four colors, namely cyan (C), magenta (M), yellow (Y) and black (K), or in six colors, namely the above-mentioned four colors and, in addition, light cyan (lc) and light magenta (lm). In order to print in color, therefore, two operations are necessary, namely an operation in which representation based upon the three primary colors RGB is converted to representation by, e.g., CMYKlclm, and an operation in which continuous tone representation of each pixel is excluded as necessary. It should be noted that although continuous tone representation of each pixel is excluded in the latter operation, continuous tone representation of an image by error diffusion or dithering is maintained.
Though color space per se is a single space, display differs depending upon how the coordinates are taken and therefore, for the sake of convenience, such space shall be referred to below as a color-specification space that conforms to the manner in which the coordinates taken.
A conversion from RGB representation to CMYKlclm representation is not uniquely defined by conversion equations. The usual practice is to obtain the mutual corresponding relationship between the color spaces in which the respective continuous levels are adopted as the coordinates and effect the conversion sequentially utilizing the corresponding relationship. Therefore, if each of the colors RGB to undergo conversion is represented by 256 levels, then a color conversion table of about 16,700,000 (256×256×256) elements will be required. In actuality, taking efficient utilization of memory resources into consideration, a corresponding relationship regarding all coordinate values is not prepared. Rather, it suffices to prepare a corresponding relationship between sporadically set grid points as a color conversion table and obtain a corresponding relationship utilizing an interpolation operation for points located between grid points. In other words, a corresponding relationship between a color of certain coordinates in RGB color-specification space and CMYKlclm color-specification space can be obtained by, e.g., linearly interpolating the corresponding relationship between grid points that surround the above coordinates.
Such a color conversion table generally is provided in a printer driver. The number of grid points of the color conversion table included in a printer driver is decided in conformity with the individual color printer.
However, the format of the above-described color conversion table is such that table size increases rapidly if use is made of a highly accurate color conversion table, namely a color conversion table having a large number of grid points. For this reason, when it is attempted to perform a highly accurate color conversion as by a photo-direct printer that prints an image that has been recorded in an external memory such as compact flash ® (CF) card, problems arise because of hardware having a comparatively small storage capacity if use is made of a printer driver that includes a highly accuracy color conversion table.
In an effort to solve this problem, the specification of, e.g., Japanese Patent Application Laid-Open No. 10-173951 discloses a method of creating a color conversion table of small size from a full-size (256×256×256 bytes in a case where each of the colors R, G, B is composed of eight bits) color conversion table by downsampling processing. Of course, this can also be achieved by simply downsampling (e.g., eliminating every other grid) the full-size color conversion table as by averaging.
The method of creating the color conversion table described in the specification of Japanese Patent Application Laid-Open No. 10-173951 above makes it possible to create a color conversion table of small size by downsampling a full-size color conversion table. However, a color conversion table of even smaller size cannot be created by downsampling an already created color conversion table having a size smaller than the full size.
Further, if the above-described simple downsampling method is used in the downsampling of a color conversion table, the way in which the ink of the gray line is used (the way in which CMYKlclm signal values are changed over) differs from that of the color conversion table prior to downsampling. More specifically, with regard to the gray line of a color conversion table having 17 grids shown in FIG. 13, which has been obtained by simple downsampling of a color conversion table having 33 grids of the kind shown in FIG. 12, the method of use of ink at the portion where the continuous tone ink changes over (indicated by the arrows in FIG. 13) is obviously different from that of the color conversion table prior to the conversion. At the portion where the continuous tone ink changes over, the point at which the dark ink starts appearing shifts and a decline in image quality, which is ascribable to downsampling of the color conversion table, tends to occur. Here a case in which the above-mentioned simple interpolation method is used in a 6-color color conversion table for RGB→CMYKlclm has been described. However, there are instances where a similar decline in image quality occurs when simple interpolation is applied by the same method to a 4-color or 3-color color conversion table for RGB→CMYK or RGB→CMY, or to a 3-color color correction table for RGB→R′G′B′.